The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of articulation weight through a damaged, unstable vertebral joint.
In some cases, when a patient having a collapsed disc moves in extension (e.g., leans backward), the posterior portion of the annulus fibrosis or folding of the ligamentum flavum may further compress and extend into the spinal canal. This condition, called “spinal stenosis”, narrows the spinal canal and causes impingement of tissue upon the spinal cord and nearby nerves, thereby producing pain.
The “80/20” rule of spine biomechanics postulates that the posterior column of the spine supports about 20% of axial spinal forces. The muscles and ligaments in the posterior column experience tensile forces in normal spine anatomy. However, in some cases of disc replacement, the axial loads borne by the anterior and posterior columns become unbalanced. By developing an implant that re-introduces those natural tensile forces, the 80/20 balance is re-established.
There have been numerous attempts to provide relief for these afflictions by providing a spacer that inserts between adjacent spinous processes present in the posterior portion of the spinal column. This spacer essentially lifts the upper spinous process off of the lower spinous process, thereby relieving stenosis. In general, these interspinous implants are adapted to allow flexion movement in the patient, but resist or limit extension.
U.S. Pat. No. 6,068,630 (“Zuchermann”) discloses a spinal distraction implant that alleviates pain associated with spinal stenosis by expanding the volume in the spinal canal or neural foramen. Zuchermann discloses a plurality of implants having a body portion and lateral wings. The body portion is adapted to seat between the adjacent spinous processes, while the wings are adapted to prevent lateral movement of the body portion, thereby holding it in place between the adjacent spinous processes.
U.S. Pat. No. 5,645,599 (“Samani”) attempts to relieve spinal stenosis by essentially inserting a flexible horseshoe-shaped device between the adjacent spinous processes. Although the Samani device desirably provides a self-limiting flexibility, it nonetheless suffers from some inadequacies. For example, the Samani device does not provide for post-operative adjustment.
In sum, conventional interspinous spacers lack post-operative adjustability.